Device for measuring the pressure of a fluid

ABSTRACT

A pressure sensing device for measuring the pressure of a fluid, comprising: a measurement diaphragm which is at least partially made of semiconductor material, is provided with a first surface and a second surface which are exposed respectively to a first pressure and to a second pressure, and undergoes a deformation following the application of the first pressure and of the second pressure; and a resonant element made of semiconductor material which is provided with a first end portion and with a second end portion for mechanically coupling the resonant element to the measurement diaphragm, the oscillation frequency of the resonant element varying according to the deformation to which the measurement diaphragm is subjected; and first circuit means for generating a sensing signal which is indicative of the oscillation frequency of the resonant element. Its particularity consisting of the fact that the resonant element comprises second circuit means for increasing the intensity of the sensing signal which is indicative of the oscillation frequency of the resonant element, the second circuit means being integrated with the structure of the resonant element.

[0001] The present invention relates to a pressure sensing device formeasuring the pressure of a fluid. More particularly, the presentinvention relates to a pressure sensing device, which is particularlysuitable for the use in industrial-type (absolute, differential orgauge) pressure measuring apparatuses, such as, for example, pressuretransmitters and the like.

[0002] It is known that various kinds of devices for sensing thepressure of a fluid have been developed in the state of the art. Thesedevices are based on different pressure transduction principles. Forexample, pressure sensing devices that are of the capacitive type,piezoresistive type, optical type, and the like are widely known.

[0003] Also, it is known that, recently, pressure sensing devices madeof semiconductor material, for example silicon, have been developed. Ingeneral, pressure sensing devices of this type are realized adoptingso-called “silicon micromachining” technologies, which allow to obtaintwo-dimensional or three-dimensional semiconductor structures withmechanical properties that can be well defined during design, despitetheir extremely small size (down to a few tens of microns). Accordingly,these structures are capable of measuring/transducing a mechanicalquantity (for example the pressure of a fluid) with high accuracy, whilemaintaining the advantages, in terms of repeatability and reliability,that are typical of integrated circuits. In the field pressure sensingdevices, made of semiconductor materials, the so-called “resonant-type”pressure sensing devices have become widespread in the industrial field.These devices have considerable advantages, such as, for example, highaccuracy and stability of measurement even for very wide measurementranges (up to several hundred bars).

[0004] A known resonant-type pressure sensing device, made ofsemiconductor material, is shown schematically in FIG. 1.

[0005] A structural element 1, made of semiconductor material, is freeto oscillate in a state of mechanical resonance. The resonant element 1is generally connected mechanically to another semiconductor structure 2(for example a diaphragm), which is capable of undergoing a deformation(indicated by the arrow 3), following the application of a pressure P tobe measured. The deformation 3 of the diaphragm 2 entails theapplication of mechanical stresses to the resonant element 1, whichaccordingly varies its own mechanical resonance frequency. Detecting, bymeans of an electronic circuit 10, the mechanical resonance frequency ofthe resonant element 1 allows obtaining a frequency-variable signal,which is indicative of the value of the pressure of the fluid. In thecommon practice, an electronic circuit of the so-called “bridge type”,such as the one shown in FIG. 2 (reference 10 a), is used to detect theresonance frequency of the resonant element 1.

[0006] The bridge circuit 10 _(a) comprises three balanced resistiveelements (R1, R2 and R3) and the resonant element 1, which has, instatic conditions, an equivalent resistance R4, whose value basicallydepends on the semiconductor material and the structure of the resonantelement 1. The bridge circuit 10 _(a) is biased, at the terminals 11 and12, with a bias voltage V_(P) whose value can also reach values ofseveral tens of volts, according to the needs. The oscillating state ofthe resonant element 1 entails a periodic variation of its ownequivalent resistance R4. This is due to the fact that the semiconductorresonant element 1 behaves as a piezoresistive element. Thus, it variesits own resistivity, if subjected to the reversible mechanicalflexural/compression stresses that are typical of the state ofmechanical resonance.

[0007] The periodic variation of the resistance R4 provokes, owing tothe balancing of the bridge circuit 10 _(a), the generation of a voltageimbalance signal V_(S) at the terminals 13 and 14 of the bridge circuit10 _(a). The imbalance signal V_(S) comprises the overlap of acontinuous voltage signal V_(C), proportional to the bias voltage V_(P),and an alternating voltage signal V_(R), whose frequency is equal to theresonance frequency of the resonant element 1. The signal V_(R)represents a sensing signal, which is indicative of the resonancefrequency of the resonant element 1. Each variation of the resonancefrequency of the resonant element 1, caused by the application of apressure P to the diaphragm 2, leads to a variation in the frequency ofthe signal V_(R). Thus, the signal V_(R) is also indicative of thepressure P, applied to the diaphragm 2.

[0008] In order to detect the resonance frequency of the resonantelement 1, other sensing circuits are known. One of these is describedin U.S. patent application No. 5,275,055 and is schematicallyrepresented in FIG. 3 (reference 10 _(b)). A voltage divider 8 iscoupled to the resonant element 1, by means of a direct electricalconnection or a piezoresistive element 7 (dashed line of FIG. 3). Alsoin this case, the signal V_(S), in output from the sensing circuit 10_(b), comprises a continuous signal V_(C), proportional to the biasvoltage V_(P), and an alternating sensing signal V_(R). The alternatingsignal V_(R) is due to the variation in equivalent resistance of theresonant element 1, in a state of mechanical oscillation, or due to thevariation in equivalent resistance of the piezoresistive element 7,associated with the resonant element 1.

[0009] Known resonant-type pressure sensing devices, despite of someundisputed advantages in terms of accuracy and stability, havedrawbacks.

[0010] Practice has shown that the sensing signal V_(R) has often a verylow intensity. In fact, often, the signal V_(R) can have a peak-to-peakamplitude of a few tens of microvolts, even for a bias voltage V_(P) ofseveral tens of volts.

[0011] This is due to the fact that the resonant element 1 has,inherently, a very high equivalent resistance. In fact, the mechanicalstructure of the resonant element 1 can be relatively complicated, sinceit is essentially aimed at enhancing as much as possible, for an equalvariation in the applied pressure P, the corresponding variation in theresonance frequency. For example, one structure, which is typically usedin the state of the art, is the so-called DETF (Double Ended TuningFork) structure, shown schematically in FIG. 4. According to thisstructure, the resonant element 1 comprises two oscillating arms 17 and18. In order to optimize mechanical performance, the arms 17 and 18 mayhave a very small thickness S and width L (a few microns) and arelatively significant length 1 (hundreds of microns). This means thatthe equivalent resistance for this type of resonant element made ofsilicon can easily reach relatively high values (tens of MOhms). This itoften happens that the percentage variation of the equivalent resistance(R4) of the resonant element 1, in a state of mechanical resonance, isrelatively low. Therefore, also the signal V_(R) has necessarily arelatively low intensity, since the voltage V_(P) generally does notassume excessively high values, in order to avoid an excessive powerdissipation.

[0012] On the other hand, also the use of a piezoresistive element,associated with the resonant element 1, has not proven to be asatisfactory solution to this problem. The practice has shown that itcan be difficult to mechanically couple the piezoresistive element tothe structure of the resonant element. In fact, this mechanical couplingoften entails damping phenomena. This means that theflexural/compression stresses, to which the structure of the resonantelement is subjected, are not optimally transmitted to the resonantelement. This entails a reduced intensity for the sensing signal V_(R).The reduced intensity of the signal V_(R) makes necessary the use ofrelatively complicated auxiliary electronics (reference numeral 19 inFIG. 1), in order to obtain a useful sensing signal V_(U) to send ininput to the electronics 16 of the pressure transmitter.

[0013] Furthermore, due to its low intensity, the sensing signal V_(R)can be greatly affected by external electromagnetic noise, which canhave an amplitude comparable to that one of the signal V_(R). This cancompromise the accuracy of the measurement. Therefore, it is oftennecessary to provide an appropriate shielding in order to avoidelectromagnetic interference. Clearly, all these drawbacks lead to anincrease in the manufacturing and operating costs of the entire pressuretransmitter.

[0014] Therefore, the aim of the present invention is to provide aresonant-type pressure sensing device, for measuring the pressure of afluid, which allows obtaining a sensing signal (such as the mentionedelectrical signal V_(R)), indicative of the oscillation frequency of theresonant element, which is provided with a relatively high intensity.

[0015] Within the scope of this aim, an object of the present inventionis to provide a resonant-type pressure sensing device, which allowsavoiding the use of auxiliary electronics for the preliminary processingof the sensing signal.

[0016] Another object of the present invention is to provide aresonant-type pressure sensing device, which can be easily produced bymeans of so-called “silicon micromachining” technologies and atrelatively low cost.

[0017] Thus, the present invention provides a pressure sensing devicefor measuring the pressure of a fluid, which comprises:

[0018] a measurement diaphragm, which is at least partially made ofsemiconductor material; the measurement diaphragm is provided with afirst surface and a second surface that are exposed respectively to afirst pressure and to a second pressure; the measurement diaphragm issubjected to a deformation, following the application of said firstpressure and of said second pressure; and

[0019] a resonant element, at least partially made of semiconductormaterial; the resonant element is provided with a first end portion andwith a second end portion for mechanically coupling the resonant elementto the measurement diaphragm, the oscillation frequency of the resonantelement varying according to the deformation to which, the measurementdiaphragm is subjected; and

[0020] first circuit means for generating a sensing signal, which isindicative of the oscillation frequency of the resonant element.

[0021] The pressure sensing device, according to the present invention,is characterized in that the resonant element comprises second circuitmeans for increasing the intensity of the sensing signal, indicative ofthe oscillation frequency of the resonant element. The second circuitmeans are at least partially integrated with the structure of saidresonant element.

[0022] The pressure sensing device, according to the present invention,entails considerable advantages. In particular, the second circuit meansallow obtaining a sensing signal, of relatively high intensity. Asexplained in details hereinafter, the second circuit means, despite ofbeing, at least partially, integrated in the physical structure of theresonant element, inherently act as a stage for the amplification of thesensing signal generated by the first circuit means. In this manner itis possible to avoid the use of auxiliary electronics for thepreliminary processing of the sensing signal.

[0023] Further characteristics and advantages of the pressure sensingdevice, according to the present invention, will be described in greaterdetail hereinafter with particular reference to the accompanyingdrawings, wherein:

[0024]FIG. 1 is a schematic view of a known pressure sensing device;

[0025]FIG. 2 is a schematic view of an electronic circuit for sensingthe resonance frequency in a known pressure sensing device; and

[0026]FIG. 3 is a schematic view of another electronic circuit forsensing the resonance frequency in a known pressure sensing device; and

[0027]FIG. 4 is a schematic view of a known type of resonant element;and

[0028]FIG. 5 is a schematic view of a pressure sensing device, accordingto the present invention; and

[0029]FIG. 6 is a partial view of a first embodiment of the pressuresensing device, according to the present invention; and

[0030]FIG. 7 is a partial view of a second embodiment of a portion ofthe pressure sensing device, according to the present invention; and

[0031]FIG. 8 is a schematic view of an electronic sensing circuit, whichcan be used in the pressure sensing device, according to the presentinvention; and

[0032]FIG. 9 is a view of another preferred embodiment of the pressuresensing device, according to the present invention; and

[0033]FIG. 10 is a view of another preferred embodiment of the pressuresensing device, according to the present invention.

[0034] With reference to the above figures, the pressure sensing device(FIG. 5), according to the present invention, is generally designated bythe reference numeral 20. The pressure sensing device 20 comprises ameasurement diaphragm 21, which is made, at least partially, ofsemiconductor material. The diaphragm 21 preferably can comprise one ormore layers of silicon with positive-type doping (commonly known asP-type doping) with relatively low doping atom concentrations(approximately 10¹⁵ cm⁻³). The measurement diaphragm 21 is provided witha first surface 23 and a second surface 24, which are exposedrespectively to a first pressure P1 and to a second pressure P2. Themeasurement diaphragm 21 is subjected to a deformation, indicated by adouble arrow 25, as a consequence of the application of the pressures P1and P2.

[0035] Furthermore, the pressure sensing device 20 comprises a resonantelement 22, which is made, at least partially, of semiconductor material(for example silicon). Preferably, the resonant element 22 can be madeof positively or negatively doped silicon and can be obtained directlyfrom one of the silicon layers of the measurement diaphragm 21, adoptingproper “silicon micromachining” techniques.

[0036] The resonant element 22 can be preferably provided with a firstend portion 26 and with a second end portion 27. The end portions 26 and27 are able to mechanically couple the resonant element 22 to themeasurement diaphragm 21. In this manner, the oscillation frequency ofthe resonant element 22 can vary, according to the deformation 25undergone by the measurement diaphragm 21. Obviously, this means thatthe oscillation frequency of the resonant element 22 can vary,ultimately, according to the resulting value of the pressure P=(P1−P2)applied to the diaphragm.

[0037] The pressure sensing device 20 comprises also first circuit means(not shown in FIG. 5) for generating a sensing signal, which isindicative of the oscillation frequency of the resonant element 22.Accordingly to what described above, the sensing signal (hereinafterreferred as V_(R)) is an alternate signal, whose frequency depends onthe resulting value of the pressure P=(P1−P2) applied to the diaphragm.

[0038] A portion of the measurement diaphragm 21 and one of the endportions (the end portion 26, for example) of the resonant element 22are illustrated with reference to FIG. 6. Also, FIG. 6 partiallyillustrates the first circuit means 28, for generating the mentionedsensing signal V_(R).

[0039] In the pressure sensing device 20, according to the presentinvention, the resonant element 22 comprises, at least partially, secondcircuit means 29 for increasing the intensity of the sensing signalV_(R). The second circuit means 29 are, at least partially, integratedwith the structure of the resonant element 22. In practice, the secondcircuit means 29 can also constitute, at least partially, an integralpart of the structure (intended as the physical structure) of theresonant element 22. In particular, according to a preferred embodimentof the present invention, the second circuit means 29 can be integratedwith the structure of the first end portion 26 and/or with the structureof the second end portion 27 of the resonant element 22.

[0040] The second circuit means 29 preferably comprise a first regionmade of semiconductor material (the stippled area 290). The first region290 includes one or more layers made of piezoresistive semiconductormaterial.

[0041] The layers of piezoresistive semiconductor material of the firstregion 290 are advantageously located where the first end portion 26and/or of the second end portion 27 are subjected to the highestflexural/compression stress during the oscillation of the resonantelement 22. The arrangement of the first region 290 can therefore beeasily designed “ad hoc”, depending on the geometry of the end portions26 and 27, which may be any according to the needs. In practice, giventhe geometry of the end portions 26 and 27, it is possible to evaluate(e.g. by virtue of simulation programs) which regions of the endportions 26 and 27 are subjected to the highest compression/flexuralstress. Then, during the manufacture of the pressure sensing device 20,using appropriate silicon micromachining techniques, it is possible tointegrate these layers of piezoresistive semiconductor material. In thismanner, the first region 290 is arranged, so as to include, at leastpartially, the regions of the end portions 26 and 27, which aresubjected to the highest compression/flexural stresses. This allows thesecond circuit means 29 to improve the amplification of the sensingsignal V_(R).

[0042] Preferably, the first region 290 can comprise one or more layersof monocrystalline silicon, whose doping concentrations can besubstantially different from the doping concentrations used for theremaining portion of the structure of the resonant element 22. Forexample, according to a preferred embodiment, the region 290 can havehigh N-type doping concentrations (for example higher than 10¹⁸ cm⁻³),while the resonant element 22 can have low P-type doping concentrations(for example lower than 10¹³ cm⁻³). Obviously, other types of dopingconcentrations may be used, according to the needs. Alternatively, thefirst region 290 can comprise one or more layers of polycrystallinesilicon, which might be opportunely doped, according to what describedabove.

[0043] Preferably, the first circuit means 28 can comprise a firstterminal 34 and a second terminal 33, which can be preferablyintegrated, at least partially, with the structure of the diaphragm 21.In this case, the terminals 33 and 34 can be electrically connected,respectively by virtue of a first conducting path 32 and a secondconducting path 31, to a second region 300 (the shaded area in FIG. 6),electrically connected to the second circuit means 29. The second region300 operates as a first sensor element 30, suitable to generate thesensing signal V_(R), indicative of the oscillation frequency of theresonant element 22. The conducting paths 31 and 32 also are preferablyat least partially integrated with the structure of the diaphragm 21 andcan be provided by silicon layers with high-concentration N-type dopingor by means of metallic layers deposited onto the diaphragm 21. Thesecond region 300 can be integrated, at least partially, with thestructure of the first end portion 26 of the resonant element 22 (seethe reference numeral 30 b of FIG. 6). The second region 300 can also beintegrated, at least partially with the structure of a portion of themeasurement diaphragm 21, which is located proximate to the first endportion 26 (see the reference numeral 30 a of FIG. 6). The second region300 can be advantageously made of piezoresistive semiconductor material(for example silicon with P-type doping). In practice it is possible touse the same type of semiconductor material used to provide the resonantelement and/or the diaphragm.

[0044] Preferably, in accordance with an embodiment of the pressuresensing device 20, according to the present invention (see FIG. 9), thefirst circuit means 28 can also comprise a third terminal 38 and afourth terminal 39. Terminals 38 and 39 can be preferably integrated, atleast partially, with the structure of the diaphragm 21) and can beelectrically connected, respectively by means of a third conducting path380 and by means of a fourth conducting path 390, to a third region (notshown), electrically connected to the first means 29. Again, paths 380and 390 can be preferably integrated, at least partially, with thestructure of the diaphragm 21. The third region operates as a secondsensor element 280, suitable for the generation of the sensing signalV_(R). The third region can be at least partially integrated with thestructure of the second end portion 27 and/or with the structure of aportion of the measurement diaphragm 21, located proximate to the secondend portion 27.

[0045] The advantages of the described embodiments related to thepressure sensing device 20 according to the invention are considerable.

[0046] If S is the flexural/compression stress, to which the resonantelement 22 is subjected, one in fact can obtain:

S=St+≈St  (1)

[0047] where St is the compression/flexural stress affecting the endportions 26 and 27 of the resonant element 22 and Sb is thecompression/flexural stress, affecting the part of the resonant elementthat does not comprise the end portions 26 and 27.

[0048] If A_(VR) is the amplitude of the signal V_(R), one can write:

A _(VR) =T*(St+Sb),  (2)

[0049] where T is the overall transduction coefficient of the sensingsystem.

[0050] Finally, since St>>Sb, one can write

A_(VR) =T*(St+Sb)≈T*St=(T1+T2)*St  (2)

[0051] where T1, T2 are two constants which are proportional to thetransduction coefficients of the first circuit means 28 and of thesecond circuit means 29. From relation (3), one can see that the effectof the second circuit means 29 is to increase the overall transductioncoefficient of the sensing system and ultimately, for an equalcompression/flexural stress affecting the end portions 26 and 27, theamplitude of the sensing signal V_(R).

[0052] The arrangement and type of doping of the first region 290 allowto further increase the value of the constant T2.

[0053] The use of different types of doping for the first region 290,the second region 300 and the remaining portions of the resonant element22 allows limiting the onset of dissipative phenomena (for exampleparasitic currents). The flow of bias current through the resonantelement 22 (the bias current preferably follows the path indicated bythe arrows 301) can also be reduced to negligible values.

[0054] In FIG. 6, the regions 290 and 300 are shown to be complementaryin covering the surface of the end portion 26. According to the needs,both regions can cover the entire surface of the end portion 26 and canbe arranged on overlapping planes. This might be advantageous if onefinds that the entire end portion 26 is subjected to a relatively highcompression/flexural stress.

[0055] As mentioned above, together with the type of doping and with thearrangement of the first region 290, the geometry of the end portion 26and/or 27 can be designed “ad hoc”. This allows not only optimizing themechanical performance of the resonant element 22 but also to allow themeans 29 to further increase the value of the constant T2 and,consequently, the intensity of is the sensing signal V_(R).

[0056] For example, with reference to FIG. 7, at least one of the twoend portions 26 and/or 27 can have a substantially H-shaped geometry. Inthis case, the first region 290 can be located on the resonant element22, while the second region 300 can be located on the diaphragm 21.Advantageously, the first region 290 can comprise a layer of siliconwith low P-type doping, while the remaining structure of the resonantelement 22 comprises layers of silicon with high N-type doping and isbiased at ground voltage. By virtue of this geometry, one can obtain aparticularly high concentration of mechanical stresses at the firstregion 290, with a consequent increase in the constant T2.

[0057] With reference to FIG. 8, the pressure sensing device 20,according to the invention, can comprise an electronic bridge circuit 40(whose structure is substantially similar to the structure of FIG. 2).The bridge circuit 40 can be electrically connected to the firstelectronic means 28, respectively by means of the first terminal 33 andthe second terminal 34 or by means of the third terminal 38 and thefourth terminal 39. The operation of the bridge circuit is ensured bythe fact that, following the application of a bias voltage to one pairof terminals, the current trends to pass through the resistive circuit41. In practice, the bias current path is indicated by arrow 301 ofFIGS. 6 and 7. As it is possible to notice, the bias current path 301 islocated at the first end portion 26 and substantially comprises thefirst circuit means 28 and the second circuit means 29.

[0058] As mentioned, the bias current does not pass substantiallythrough the remaining body of the resonant element 22, whose equivalentresistance (R5) is relatively much higher than the circuit 41. Thebridge circuit 40 can be electrically connected to the first electronicmeans 28 by means of the third terminal 38 and the fourth terminal 39.In this case, the bias current can pass also through a resistive circuit42, which is located at the second end portion 27 and substantiallycomprises the first circuit means 28 and the second circuit means 29.

[0059] In a preferred embodiment of the pressure sensing device,according to the present invention, shown in FIG. 9, the measurementdiaphragm 21 comprises a first portion 51 and a second portion 52, madeof semiconductor material, which are mutually separated, so as to form agap 53 in between. The portions 51 and 52 are preferably electricallyinsulated from each other. The resonant element 22 is arranged, so thatit can be accommodated in the gap 53 and preferably comprises one ormore oscillating arms 54, arranged between the first end portion 26 andthe second end portion 27. According to the embodiment of FIG. 9, theoscillating arms 54 of the resonant element 22 are arranged, so thatthey are substantially parallel to the walls of the gap 53. In thiscase, the terminals 34 and 38 of FIG. 8 are comprised within the portion51 of the measurement diaphragm 21, while the terminals 33 and 39 ofFIG. 8 are comprised within the portion 52 of the measurement diaphragm21.

[0060] Alternatively, as shown in FIG. 10, the oscillating arms 54 ofthe resonant element 22 are arranged, so that they are substantiallyperpendicular to the walls of the gap 53. In this case, the terminals 33and 34 of FIG. 8 are comprised within the portion 51 of the measurementdiaphragm 21, while the terminals 38 and 39 of FIG. 8 are comprisedwithin the portion 52 of the measurement diaphragm 21.

[0061] Preferably, the resonant element 22 comprises two arms 54 whichoscillate in a direction of motion, which is substantially parallel(arrow 55) to the surface of the measurement diaphragm 21.

[0062] The portions 51 and 52 that are electrically insulated, as wellas the gap 53, can be obtained by applying silicon micromachiningtechniques to the measurement diaphragm 21.

[0063] Advantageously, the pressure sensing device 20 can compriseexcitation means (not shown), aimed at sustaining the oscillation of theresonant element 22. These excitation means may be preferablyintegrated, at least partially, on the diaphragm 21.

[0064] In practice it has been found that the pressure sensing device,according to the present invention, fully achieves the intended aim andobjects, since in particular it has been found that it is possible toobtain amplitudes of the sensing signal V_(R), which are remarkablyhigher than in known pressure sensing devices. In particular, amplitudevalues higher by almost one order of magnitude (a few mVs) can be easilyobtained. This allows avoiding the use of auxiliary electronic circuitsfor the preliminary processing of the sensing signal. The pressuresensing device, according to the present invention, has furthermoreproven to be easy to manufacture with known silicon micromachiningtechniques, thus allowing reducing significantly the manufacturing andinstallation costs of the pressure transmitter, in which the pressuresensing device can be commonly used. The pressure sensing device,according to the present invention, is particularly suitable forpressure measurements of the differential type. Also measurements of theabsolute type and the gauge type can be easily obtained. In the firstcase, P1 or P2 has negligible values, closed to vacuum pressure values.In the second case, P1 or P2 has values closed to atmospheric pressurevalues.

1. A pressure sensing device, for measuring the pressure of a fluid,comprising: a measurement diaphragm, which is at least partially made ofsemiconductor material and is provided with a first surface and a secondsurface that are exposed respectively to a first pressure and to asecond pressure, said measurement diaphragm being subjected to adeformation, following the application of said first pressure and saidsecond pressure; and a resonant element, at least partially made ofsemiconductor material, which is provided with a first end portion andwith a second end portion for mechanically coupling said resonantelement to said measurement diaphragm, the oscillation frequency of saidresonant element varying as a function of the deformation, to which saidmeasurement diaphragm is subjected; and first circuit means forgenerating a sensing signal, which is indicative of the oscillationfrequency of said resonant element; characterized in that said resonantelement comprises second circuit means for increasing the intensity ofsaid sensing signal, said second circuit means being at least partiallyintegrated with the structure of said resonant element.
 2. The pressuresensing device, according to claim 1, characterized in that said secondcircuit means are integrated with the structure of the first end portionand/or the second end portion of said resonant element.
 3. The pressuresensing device, according to one or more of the previous claims,characterized in that said second circuit means comprise a first regionof semiconductor material, which comprises one or more layers ofpiezoresistive semiconductor material.
 4. The pressure sensing device,according to one or more of the previous claims, characterized in thatsaid first region is arranged, so as to comprise, at least partially,the regions of the first end portion and/or the second end portion ofsaid resonant element, which are subjected to the highestflexural/compression stress during the oscillation of said resonantelement.
 5. The pressure sensing device, according to one or more of theprevious claims, characterized in that said first circuit means comprisea first terminal and a second terminal, which are electricallyconnected, respectively by means of a first conducting path and a secondconducting path, to a second region, made of piezoresistivesemiconductor material.
 6. The pressure sensing device, according toclaim 5, characterized in that said second region operates as a firstsensor element, suitable to generate said sensing signal.
 7. Thepressure sensing device, according to one or more of previous claims,characterized in that said second region is, at least partially,integrated with the structure of the first end portion of said resonantelement and/or with the structure of a portion of said measurementdiaphragm, located proximate to the first end portion of said resonantelement.
 8. A pressure sensing device, according to one or more of theprevious claims, characterized in that said first circuit means comprisea third terminal and a fourth terminal which are electrically connected,respectively by means of a third conducting path and a fourth conductingpath, to a third region, made of piezoresistive semiconductor material.9. The pressure sensing device, according to claim 8, characterized inthat said third region operates as a second sensor element, suitable togenerate said sensing signal.
 10. The pressure sensing device, accordingto one or more of previous claims, characterized in that third secondregion is, at least partially, integrated with the structure of thesecond end portion of said resonant element and/or with the structure ofa portion of said measurement diaphragm, located proximate to the secondend portion of said resonant element.
 11. The pressure sensing device,according to one or more of the previous claims, characterized in thatsaid first terminal and/or said second terminal and/or said thirdterminal and/or said fourth terminal are at least partially integratedin the structure of said measurement diaphragm.
 12. The pressure sensingdevice, according to one or more of the previous claims, characterizedin that said first conducting path and/or said second conducting pathand/or said third conducting path and/or said fourth conducting path areat least partially integrated in the structure of said measurementdiaphragm.
 13. The pressure sensing device, according to one or more ofthe previous claims, characterized in that said resonant elementcomprises a pair of arms, which oscillate in a direction of motion whichis substantially parallel to the surface of said measurement diaphragm.14. An absolute pressure measuring apparatus, such as an absolutepressure transmitter or the like, characterized in that it comprises apressure sensing device, according to one or more of claims 1 to
 13. 15.A gauge pressure measuring apparatus, such as an gauge pressuretransmitter or the like, characterized in that it comprises a pressuresensing device, according to one or more of claims 1 to
 13. 16. Adifferential pressure measuring apparatus, such as a differentialpressure transmitter or the like, characterized in that it comprises apressure sensing device, according to one or more of claims 1 to 13.